Girls with Rett Syndrome are studied with emphasis on clinical epidemiological, biochemical, and positrol emission tomography (PET) studies. PET scans show reduction of 18F deoxyglucose uptake in the occipital region of cases studied. All EEG's and most somatosensory evoked potentials are abnormal. Negative results of chemistries, lactate pyruvate ammonia, biotinidase, carnitine, amino acids in CSF, plasma and urine as well as lysosomal studies are of note.